A mass spectrometer with tandem mass spectrometry, which dissociates a material (a parent ion) and analyzes dissociated ions, has been prevailing so as to improve the accuracy of identification of the material based on the data of mass spectrometry. Methods for identifying a parent ion and deriving the estimated structure thereof, which use mass spectrometric data, MS data for the parent ion and MS/MS data for a dissociated ion, are categorized as follows:
(1) Method for retrieving database of mass spectrometric data for a parent ion (MS data)
(2) Method for retrieving database of mass spectrometric data for a parent ion and a dissociated ion (MS data and MS/MS data)
(3) Method for employing measured mass spectrometric data for a parent ion and a dissociated ion (MS data and MS/MS data) but not utilizing database retrieval
As an example of the conventional method (2), the Japanese Published Patent Application 8-124519 discloses a method for determining a candidate for parent ion. The method has the steps of: picking up candidates for an ion species, which have peaks correlating with those of mass spectrum of the ion species respectively, referring to a database of peaks; picking up candidates for a desorptive base which have desorptive masses correlating with those of the ion species, referring to a database of desorptive bases; and determining a candidate for a parent ion referring to a database which stores regulations applied to construction of the parent ion from dissociated ions and desorptive bases.
Also as an example of the conventional method (3), there is a computer program called “SeqMS” for supporting an analysis for amino acid sequence developed by Osaka University in Japan, which is reported in Lectures on Experiment in Post-Genome Era pages 137 to 139. The computer program is able to support in identifying amino acid sequences for a peptide without database retrieval, which includes about ten amino acid sequences. The method applied to the program, which employs statistical processing based on graph theory that takes into account a weighted value of dissociation probability empirically obtained from the mass spectrometric data of a peptide ion and dissociated ions thereof, provides candidates for the amino acid sequence.
Since data is not stored in a database for materials having unknown structure, it is difficult to identify a parent ion and derive estimated structure for a material having unknown structure if an analysis is performed with conventional methods (1) and (2) using database retrieval. These methods employ mass spectrometric data (MS data) for a parent ion and mass spectrometric data (MS/MS data, MS/MS/MS data) for a dissociated ion respectively in order to support the analysis.
Another method (3) described above, which employs statistical processing and information processing based on numerical matching, currently provides poor accuracy less than 50 percent in identifying a parent ion.